The chemical vapor deposition method (CVD) is an art for precipitating a reaction product in the form of a film on the surface of a substrate based on the vapor-phase growth in a high-temperature atmosphere by using a starting gas which does not undergo the reaction at normal temperature, has been widely employed for the production of semiconductors and for reforming the surfaces of metals and ceramics, and has, in recent years, been also used for reforming the surfaces of plastic containers and, particularly, for improving gas-barrier property.
The plasma CVD is a process for growing a thin film by utilizing a plasma and basically comprises decomposing a gas containing a starting gas by an electric discharge of electric energy in a high electric field under a reduced pressure, and depositing a formed substance on the substrate through a chemical reaction in the vapor phase or on the substrate.
The plasma state is realized by a glow discharge. Depending upon the system of glow discharge, there have been known a method that utilizes a DC glow discharge, a method that utilizes a high-frequency glow discharge and a method that utilizes a microwave glow discharge.
There has been known a method of forming a silicon oxide film (SiOx) on the surface of a substrate such as a plastic container based on the plasma CVD method (see, for example, JP-A-2000-255579).
According to the method disclosed in the above prior document, a silicon oxide film is formed on the inner surface of the plastic container relying on a plasma treatment by a glow discharge by feeding a mixed gas of an organosilicon compound gas and an oxidizing gas into a plasma-treating chamber, wherein a feature resides in that the concentration of the organosilicon compound gas in the mixed gas fed into the plasma-treating chamber is varied while forming the film by the plasma treatment, offering an advantage in that the facility can be simplified in the exhaust system in the plasma-treating apparatus and the film can be formed in a shortened period of time.
The above method teaches the following means (a) to (c) for varying the concentration of the organosilicon compound gas in the mixed gas while forming the film.
(a) The amount of feeding the organosilicon compound gas is decreased (or decrease and increase are repeated) while forming the film;
(b) Feeding of the organosilicon compound gas itself is interrupted (or is interrupted and started again, or stop and start are repeated) while forming the film; and
(c) The flow rate of the organosilicon compound gas and the flow rate of the oxygen gas (or gas having an oxidizing power) are both varied.
However, the silicon oxide film formed by the above method has a problem in that it lacks softness and flexibility, and adheres poorly to the substrate. When the substrate is a plastic material, in particular, this tendency becomes strong and the film tends to be broken. For example, when the hot water is preserved in a plastic container having a silicon oxide film formed on the inner surface thereof, the film is broken by the deformation of the container due to reduced pressure and even by a slight contraction of the container (though no problem is caused to the container in practice) due to the heat, making it difficult to obtain gas-barrier property and flavor-retaining property as desired.
According to this method which varies the concentration of the organosilicon compound gas itself, which is a source of feeding silicon that is an important element for constituting the vapor deposited film, gas-barrier property-tends to be dispersed when many substrates are treated making it difficult to obtain a predetermined gas-barrier property maintaining stability. This tendency becomes particularly conspicuous when a silicon oxide film having a small thickness yet a high performance is to be obtained through the treatment with a plasma in a short period of time.
Further, the conventional vapor deposited films formed by the above method have high barrier property against various gases (e.g., oxygen), but also exhibit high water-permeating property permitting water to infiltrate through the vacuum evaporated film and, hence, exhibiting decreased gas-barrier property. Therefore, it has been urged to improve the properties especially in the field of packaging materials.